Not Applicable
Not Applicable
1. Field of the Invention
The present invention is in the field of downhole jarring devices used in oil and gas well drilling and downhole equipment recovery. More specifically, it is a device that imparts rapid impacts to the desired portion of the work string or a stuck object, often referred to as a xe2x80x9cfishxe2x80x9d, for the purpose of loosening the stuck object.
2. Background Art
In well operation, there is often a need for jarring, impact or vibration devices to move downhole stuck members. Jars are typically included in a pipe or work string to provide upward or downward impacts when activated. Jars are usually single impact devices which must be recocked each time before operation, limiting the number of impacts per minute, and therefore limiting the energy, that can be delivered to a stuck member.
Some known impact tools require the operator to pull up on the work string with a force sufficient to pre-stress the work string, thereby providing the motive force for an impact. The impact is typically initiated when some type of valve or other triggering device in the tool triggers an action which applies the energy stored in the pre-stressed work string in the form of an impact delivered to the stuck object. The force of the impact delivered by such a tool depends upon how much energy is stored in the pre-stressed work string. That is, a larger over-pull will deliver a harder blow to the stuck object.
Often, in the use of this type of tool, the weight of the fish itself can be significant enough to raise the tension in the work string to such a high level that the tool will cease to function. More specifically, the force which can be applied to the triggering device by the flow of fluid is limited by the available fluid flow rate. The higher the amount of pre-stress tension, the harder it is to make the tool function. If the weight of the fish is too close to the pre-stress limit of the tool, the tool will cease to function as the fish begins to loosen. The operator then has to reduce the pre-stress on the work string to make the tool resume functioning, thereby limiting the force available in each impact and making the tool less effective.
Further, a tool which relies on work string pre-stress often has a fluid flow path which allows well bore return fluid to enter the tool, which exposes the internal tool parts to well bore debris. This can clog or restrict the moving parts and render the tool inoperative, it can cause failure of the seals, or it can cause the tool to wear out prematurely.
The device of the present invention uses hydraulic power from surface pumps to repeatedly compress an internal piston spring in the tool. The piston spring is repeatedly allowed to expand, to deliver continuous rapid impacts. The sustained energy that is delivered to the stuck member becomes a motivating force to free the stuck member. When the operator desires, the fluid flow rate through the tool is increased to a selected level, which will exert sufficient hydraulic pressure to move a dart valve to seal against a valve seat on a flow-through piston. This cuts off flow through the piston and drives the piston and the dart valve downwardly. As the piston moves downwardly, it compresses the piston spring. At a designed tripping point, the dart valve is lifted away from the valve seat on the piston by a tripping spring, allowing flow through the piston to resume, sharply decreasing the hydraulic pressure on the piston. This allows the piston spring to drive the piston sharply upward, delivering an impact to the tool housing. Movement of the dart valve away from the piston seat is arrested by a momentary increase in hydraulic pressure above the dart valve, caused by a momentary cutoff of fluid flow through the dart valve. The dart valve is then driven downwardly again, and the cycle repeats rapidly.
The motive force for the impact is generated entirely within the tool, eliminating any need for prestressing the tool from above. This allows the tool to function regardless of the weight of the stuck object. No return fluid flow passes through the tool, eliminating the danger of contamination by well debris.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: